The present invention relates to an image forming apparatus for developing an electrostatic latent image formed on a photosensitive drum by a toner while rotating the photosensitive drum, transferring the toner image to paper, and fixing the toner image on the paper and, more particularly, to an image forming apparatus for forming an image by smoothly rotating a photosensitive drum by a stepping motor.
As a driving system for driving a stepping motor, there are a one-phase excitation system, a one-two phase excitation system and a two-phase excitation system. For example, in a two-phase excitation system for a stepping motor provided with four-phase exciting coils (phase A, phase *A, phase B, phase *B), (1) excitation of the phases A and B, (2) excitation of the phases B and *A, (3) excitation of the phases *A and *B, (4) excitation of the phases *B and A are repeated in series, thereby rotating the rotor of a step motor at an electrical angle of 90 degrees per excitation, as shown in FIG. 11. In FIG. 11, the symbols SA, SB, *SA, *SB represent the driving signals in the phases A, B, *A and *B, respectively.
FIG. 12 shows the structure of an excitation circuit for driving a stepping motor by a two-phase excitation system. In FIG. 12, the reference numeral 1 represents a stepping motor, 1a a coil in the phase A, 1b a coil in the phase *A, 1c a coil in the phase B, and 1d a coil in the phase *B. The reference numeral 2 represents an excitation circuit for the phases A and *A and 3 an excitation circuit for the phases B and *B. The excitation circuits 2 and 3 have the same structure. The reference numeral 4 denotes a reference voltage generator, which divides a constant terminal voltage of a Zener diode 4a by a variable resistor 4b and a fixed resistor 4c and inputs a reference voltage V.sub.REF which corresponds to a reference current (constant) into each of the excitation circuits 2, 3.
In the excitation circuit 2, the reference numerals 2a, 2b denote switching transistors (which may be FET, etc.) connected in series to the coils 1a, 1b in the phases A and *A, 2c a detector resistor for detecting the exciting currents I.sub.A, *I.sub.A which flow on the coils 1a, 1b when the corresponding switching transistors 2a, 2b are in "on" state, and 2d, 2e diodes for causing flyback currents I.sub.FA, *I.sub.FA to flow on the coils 1a, 1c when the corresponding switching transistors 2a, 2b are turned off. The reference numeral 2f represents a comparator for comparing a detected current value (the terminal voltage V.sub.D of the detector resistor 2c) with the reference current value (reference voltage V.sub.REF), and generating a pulse Sc when V.sub.D .gtoreq.V.sub.REF, 2g a one-shot multivibrator for outputting a signal Sd which is held at a low level for a predetermined time when the pulse Sc is output from the comparator 2f and which is elevated to a high level thereafter, 2h an AND gate for passing the output Sd of the one-shot multivibrator 2g therethrough when an A-phase driving signal SA is input, and 2i an AND gate for passing the output Sd of the one-shot multivibrator 2g therethrough when an *A-phase driving signal *SA is input. The excitation circuit 3 has the same structure as the excitation circuit 2, but it excites the coils 1c and 1d in the phases B and *B, respectively.
FIG. 13 shows the waveform of a signal in each portion of the excitation circuit 2 for exciting the coil 1a in the phase A. Since the one-shot multivibrator 2g ordinarily outputs a signal Sd of a high level, the AND gate 2h outputs a switching signal Se of a high level when the driving signal SA is elevated to a high level. The transistor 2a is then turned on, and the A-phase exciting current I.sub.A flows on the coil 1a in the phase A and the voltage V.sub.D which corresponds to the exciting current value is input to the comparator 2f. The comparator 2f compares the reference voltage V.sub.REF with the detected voltage V.sub.D, and generates a pulse Sc when V.sub.D becomes not less than V.sub.REF. When the pulse Sc is generated, the output signal Sd of the multivibrator 2g is reduced to a low level for a predetermined time Tm. As a result, the switching transistor 2a is turned off, and the exciting current I.sub.A becomes zero. When the switching transistor 2a is turned off, the energy stored in the coil 1a in the phase A flows in the diode 2d as the flyback current I.sub.FA.
When the predetermined time Tm elapses, since the output signal Sd of the multivibrator 2g is raised to a high level again, the switching transistor 2a is turned on, and the exciting current I.sub.A begins to flow again. This operation is repeated during the time in which the phase A driving signal SA is held at a high level, and the exciting current I.sub.AA (=I.sub.A +I.sub.FA) flows on the coil 1a in the phase A in total. The exciting current I.sub.AA has a substantially rectangular shape.
In this manner, substantially rectangular exciting currents I.sub.AA, *I.sub.AA, I.sub.BB, *I.sub.BB flow on the respective coils while the driving signals SA, *SA, SB, *SB are at high levels, thereby sequentially rotating the stepping motor.
In the above-described stepping motor driving system, however, since the stepping motor is rotated at 90 degrees each time, the rotation is step-wise, so that minute vibration generates. When a stepping motor is used as a driving source of a photosensitive drum of an image forming apparatus such as an electrophotographic printer and a copying machine, the minute vibration deteriorates an image. For this reason, a stepping motor is not used as the driving source of a photosensitive drum of an image forming apparatus, and a DC motor or a brushless DC motor is used instead. However, it is now necessary to drive a photosensitive drum by a stepping motor in response to the recent demand for an inexpensive, small-sized high-speed image forming apparatus having a long life.
Especially, in color printers, a photosensitive drum for a black color and three photosensitive drums for the three primary colors are necessary. In addition, since it is required in color printing to sequentially change toners so as to correctly superimpose different colors, it is necessary to rotate the photosensitive drums with high accuracy. It is also necessary to drive the belt with high accuracy so as to feed paper to the toner transferring position of each photosensitive drum. In this way, in color printers, it is necessary to drive a photosensitive drum and a belt by a small-sized motor which is produced at a low cost and which enables high-speed and highly accurate driving. To meet such demand, it is necessary to drive a photosensitive drum by a stepping motor.